1. Field of the Invention
Exemplary embodiments of the present invention relate to a spark plug or igniter for an internal combustion engine, and more particularly to compositions for and methods of making an insulator for a spark plug or igniter.
2. Description of the Background
As is illustrated in FIG. 1, a conventional spark plug 10 may include an annular metal casing or shell 12 having a cylindrical base 14 with external threads 16 formed thereon for threadable engagement in a cylinder head (not shown) of an internal combustion engine. The cylindrical base 14 of the spark plug shell 12 may have a generally flattened lower surface 18. A ground or side electrode 20, for example, of a noble metal, may be welded or otherwise attached to the lower surface 18 of the threaded base 14. An electrode tip 22 may be welded or otherwise attached to an end of the side electrode 20.
The spark plug 10 may further include a hollow ceramic insulator 24 disposed concentrically within the shell 12 and a center electrode 26 disposed concentrically within the insulator 24. The center electrode 26 may include a central core 28 that is made of a thermally and electrically conductive material and an outer cladding 30.
An electrically conductive insert or rod 36 fits into an upper end 38 of the insulator 24 opposite the center electrode 26 and a refractory glass-carbon composite material is disposed within the insulator 24 between a lower end 39 of the insert 36 and the center electrode 26 to provide an internal resistor 40 with the spark plug 10.
As illustrated in FIG. 1, the spark plug shell 12 is a substantially cylindrical sleeve having a hollow bore 42 formed therethrough. As noted above, the spark plug shell 12 includes the cylindrical base portion 14, which generally has threads 16 formed on the exterior surface thereof. The spark plug shell 12 may include a sealing surface 44 for contacting the cylinder head (not shown) and, on the spark plug shell 12 above the sealing surface 44, a generally hexagonal boss 46 for allowing the spark plug 10 to be grasped and turned by a conventional spark plug socket wrench for installation or removal thereof.
The insulator 24 is a ceramic article that has been conventionally manufactured, as depicted in FIG. 2, by collecting the raw materials necessary to form an insulator blank at block 102, preparing a raw material based powder by blending the appropriate percentages of each of the raw materials to create a desired powdered formulation at block 104, and spray drying the powdered formulation at block 106. A blank is thereafter formed by pressing the spray-dried powder at block 108, grinding or green machining the pressed blank on a grinding wheel to form an insulator preform at block 110, and firing or sintering the insulator preform to a high temperature sufficient to densify the preform and sinter the powder particles to form a finished insulator or chalk insulator at block 112. The insulator preform is generally fired at temperatures of up to between about 1400 and about 1600 degrees Celsius. A prior art sintering profile is depicted in FIG. 3, wherein a peak temperature of a little less than 1600 degrees Celsius is reached.
Following the trend of engine downsizing, spark plugs have become thinner and longer. In order to fit in the smaller package, the ceramic insulators of such spark plugs have been significantly reduced, leading to a reduction in maximum ignition voltage the spark plug can withstand. However, as the result of engine downsizing and the wider use of turbocharging, higher cylinder pressures are anticipated for future combustion engines, which require higher ignition voltages and higher operation temperature. These challenges have demanded that insulators of spark plugs for future combustion engine possess much higher dielectric strength than those used today.